The wear resistance and surface roughening resistance of a roll are important properties determining the productivity of rolling. Poor wear resistance causes a roll surface to be prematurely worn, resulting in rolled products with deteriorated dimensional precision. Also, when a roll surface is non-uniformly worn and roughened by contact with a work, a back-up roll, etc., such surface roughness is transferred to a work surface, resulting in a work with deteriorated appearance. To prevent this, the roll should be exchanged frequently, resulting in frequent stop of a rolling operation and thus decrease in the productivity of rolling factory, cost increase by the cutting of a roll surface, and decrease in yield consumption by the increased cutting of a roll surface.
The seizure resistance of a roll is also important. Poor seizure resistance causes a work to be seized with the roll by heat generated in the roll bite, etc. during rolling, failing to conduct normal rolling. Particularly in a downstream stand at a finishing stage of a hot strip mill, overlapped end portions of two works are rolled for some reasons, causing an accident called “thickness reduction.” In this case, if the roll has poor seizure resistance, the work may be seized with the roll and wound around the roll body, making rolling stop inevitable. If rolling continues with the work seized with the roll, a rolling load is concentrated in a seized portion, generating cracks, from which spalling, etc. occurs to cause fracture.
The harder the roll, the higher wear resistance it has. A high-speed steel roll material contains high-hardness carbides of alloying elements such as MC, M2C, M6C, M7C3, etc. Among the alloying elements, particularly V and Nb form extremely hard MC carbides having Vickers hardness Hv of about 2400-3200, remarkably contributing to improvement in wear resistance. However, when a melt containing large amounts of V and Nb is centrifugally cast, MC carbides are centrifugally segregated inward.
JP8-60289A discloses a centrifugally cast, solid or hollow, composite roll comprising an outer layer having a composition comprising, by mass, 1.0-3.0% of C, 0.1-3.0% of Si, 0.1-2.00% of Mn, 2.0-10.0% of Cr, 0.1-10.0% of Mo, 1.0-10.0% of V, 0.1-10.0% of W, Mo+W≦10.0%, and the balance being Fe and impurities, and an inner layer of cast iron or steel. This reference describes that when V exceeds 10.0% by mass, light carbides are segregated toward an inner surface by centrifugal casting, with small amounts of carbides remaining at an outer surface of an outer layer to be used for rolling. This phenomenon tends to occur when granular carbides are primarily crystallized from a melt. Because primarily crystallized granular carbides have specific gravities of about 6 g/cm3, lighter than the melt having a specific gravity of about 7-8 g/cm3, they move toward an inner surface by a centrifugal force.
To prevent segregation due to centrifugal separation by providing carbides with larger specific gravities, JP9-256108 A proposes a tool steel for hot-rolling, which comprises 3.5-5.5% of C, 0.1-1.5% of Si, 0.1-1.2% of Mn, 4.0-12.0% of Cr, 2.0-8.0% of Mo, and 12.0-18.0% of V the balance being Fe and inevitable impurities. VC having a small specific gravity is segregated by centrifugal casting, while Nb prevents the segregation of carbide by centrifugal separation because it forms composite carbide of (V; Nb)C having a large specific gravity with V.
JP3-254304A discloses a composite roll for hot-rolling, which has an outer layer having a structure containing 5-30% of granular carbide and 5% or less of non-granular carbide by area, its matrix having Vickers hardness Hv of 550 or more. The outer layer of this composite roll for hot-rolling has a basic composition comprising 1.0-3.5% of C, 3.0% or less of Si, 1.5% or less of Mn, 2-10% of Cr, 9% or less of Mo, 20% or less of W, and 2-15% of V by mass, the balance being Fe and impurities. However, this composite roll is produced by a so-called continuous casting method, by which an outer layer is continuously formed around a steel shaft using a high-frequency coil. The continuous casting method suffers from a higher production cost than the centrifugal casting method, and is little adapted for producing a large roll.
JP7-268569A discloses a wear-resistant sintered alloy comprising, by weight, 1.8-5% of C, 2% or less of Si, 2% or less of Mn, 4-6% of Cr, 2-8% of W, 2-10% of Mo, more than 11% and 17% or less of V, and 7-13% of Co, the balance being Fe and inevitable impurities, and containing MC carbide particles having an average diameter of 1-30 μm at an area ratio of 20-40%. However, because this alloy is produced by a HIP method, it suffers from a higher production cost than that produced by the centrifugal casting method, and is little adapted for producing a large roll.
JP2000-303135 A discloses a composite roll for hot-rolling, which comprises a centrifugally cast outer layer and an inner layer fused via an intermediate layer, the outer layer having a composition comprising, by weight, 1.5-2.6% of C, 0.1-2.0% of Si, 0.1-2.0% of Mn, 7-15% of Cr, 2.5-10% of Mo, 3-10% of V, and 0.5-5% of Nb, the balance being Fe and inevitable impurities, and the composite roll having an average thermal expansion coefficient of 11.5×10−6/° C. or less in a range from room temperature to 300° C. In this composite roll, thermal expansion is suppressed by controlling the thermal expansion coefficient of the outer layer. However, because MC carbides such as VC, etc. are segregated inward in the centrifugally cast outer layer, the surface wear resistance is not necessarily fully improved.